Autoantibodies against thyroid peroxidase (TPO) are present in the sera of virtually all patients with active Hashimoto's thyroiditis. Understanding the precise molecular interaction between autoantibodies and TPO is a necessary step towards understanding the basis of the autoimmune response and, ultimately, towards the goal of immunotherapy for this very common autoimmune disease. The goals of this proposal are: 1. CLONING AND EXPRESSION OF THE TPO AUTOANTIBODY GENE REPERTOIRE We have cloned a panel of human TPO autoantibodies, but hose in the repertoire remain, particularly for: . autoantibodies with linear epitopes (denatured TPO) . autoantibodies with lambda L. chains . autoantibodies whose epitopes lie outside the immunodominant region A wide repertoire of TPO autoantibodies is also required to study the role of autoantibodies in antigen presentation to T cells. We will use the combinatorial library to complete the TPO autoantibody repertoire for genes and epitopes. We will use the combinatorial library to complete the TPO autoantibody repertoire for genes and epitopes. 2. 3D STRUCTURE OF THE TPO IMMUNODOMINANT REGION AND ITS AUTOANTIBODIES Definition of the precise interaction between TPO autoantibodies and its antigen will require over-expression, purification and crystallization of selected molecules for x-ray diffraction analysis. We will study the following: . TPO autoantibody Fab to the immunodominant Fab to the immunodominant region . TPO . TPO: autoantibody Fab complexes. 3. ROLE OF AUTOANTIBODIES IN THE PRESENTATION OF TPO TO T CELLS Monoclonal human TPO autoantibodies provide unique tools for studying the influence of autoantibodies on the processing and presentation of TPO to T cells. For this purpose, we will: . Establish and characterized a panel of TPO-specific T cell clones. . Study the influence of TPO autoantibody epitopes on TPO presentation to TPO specific T cells. . Study the ability of TPO-specific T cell clones to drive TPO autoantibody production by memory B cells.